TCE Announcement
Table of Contents
Implementation of Revised Slope Factors
The Office of Land Quality (OLQ) derived draft slope factors for Trichloroethylene (TCE, CAS 79-01-6) and released the technical document supporting that derivation for comment on January 4, 2006: "A Regulatory Approach for Deriving Trichloroethylene Cancer Potency Estimates for use in the Development of Health Based Remediation Closure Levels" (Review Draft) [DOC]. If you are unable to access this document please email IDEMRISC at idem.IN.gov.
OLQ is proposing to replace the slope factors in the 2004 Default Closure Level Tables. After an evaluation of the comments, OLQ will issue revised slope factors. OLQ plans to use the revised slope factors until USEPA releases further guidance on TCE. The draft slope factors will be able to be used immediately in any nondefault application. The draft slope factors generally increase the default closure levels, especially the industrial values. The revised default closure levels are given in Table 1.
Table 1:
Draft TCE Default Closure Levels
| Soil | GW Default Closure Level mg/l | Indoor Air | |||
|---|---|---|---|---|---|
| Soil Direct mg/kg | Migration to GW mg/kg | Default Closure Level mg/kg | Chronic ug/m3 | ||
| 2006 Residential | 4.9 | 0.057 | 0.057 | 0.005 | 1.2 |
| 2006 Industrial | 24 | 0.35* | 0.35 | 0.031* | 7.9 |
*these numbers reflect the non-carcinogenic end point, which results in a moderately lower closure level than the carcinogenic end point.
Development History
Slope factors are cancer potency estimates and are used, along with other inputs, to derive closure levels at remediation sites. OLQ has derived the draft slope factors from a group of slope factors listed in a USEPA draft document issued by the National Center for Exposure Assessment (NCEA). USEPA NCEA presented a group of slope factors with values ranging from 0.02 to 0.4 (mg/kg-day)-1 and OLQ has selected values derived from studies within that range.
The release of the NCEA document and its use has created significant controversy across USEPA and the United States. NCEA not only released new science information with their document, but also presented states with new choices on how to use slope factors. The way in which OLQ has decided to interpret the NCEA document is important in understanding how and why OLQ derived the draft slope factors.
OLQ uses a hierarchy of sources to derive slope factors. OLQ established this hierarchy in the late 90s through a series of meetings with the regulated community during development of the Risk Integrated System of Closure (RISC). Two principal sources within the hierarchy are USEPA Regions 3 and 9. These two Regions publish tables of screening level values, very similar in purpose to the RISC default closure level tables.
The Region 3 and 9 tables have a lot of information, including slope factors. It has been common practice, based on the hierarchy, for OLQ to use the slope factors listed in these documents to develop closure levels. Slope factors, as with other types of toxicity information, continually change, as new science information becomes known. The Regions adopt this new information on a regular basis and OLQ, in turn, adopts the same information.
Shortly after the release of the NCEA document Regions 3 and 9 interpreted this document and adopted the high end or most conservative portion of the slope factor range for use in their tables. In 2004, OLQ updated its tables and adopted the new slope factors presented by Regions 3 and 9. As a result, the default closure levels in Indiana changed significantly, becoming much lower. The regulated community responded to these new lower closure levels and commented that they were derived from a NCEA "draft" document. Since it is common practice for the Regions to reference NCEA as a source for toxicity information when the primary source, the USEPA Integrated Risk Information System was silent, OLQ was not alarmed. OLQ recognized that the document was draft but failed to find a situation in which the new closure levels had a practical impact on clean up and also recognized that under RISC, a responsible party has the opportunity to explore different options.
However, as OLQ continued to investigate this issue it became apparent that one of the significant problems with the NCEA document was unclear guidance regarding how to use the range for site-specific applications. As OLQ was using the high end or most conservative portion of the range it seemed unreasonable not to have clear guidance on when to deviate from the high end. The USEPA Science Advisory Board, while supporting the use of a range of slope factors, reached a conclusion similar to OLQ and requested that USEPA provide clearer guidance on how to use and interpret the range of values. Shortly after, the National Academy of Science (NAS) also became involved. When the NAS became involved it became clear that new USEPA guidance would not be forthcoming in a timely manner and OLQ decided to try to resolve the issue internally.
At first, OLQ attempted to develop guidance on how to use the range, finally realizing this was not possible without further input from USEPA. OLQ also evaluated returning to the old, pre-2004, slope factors, but decided that it was not in the best interest of the public or the regulated community to do so. It seemed unreasonable to ignore the significant body of new science regarding TCE's carcinogenic potency and the hierarchy sources, Regions 3 & 9, clearly had used this document to derive their slope factors.
Finally, OLQ decided to derive a single slope factor value using the range of slope factor values and the studies used to derive them. OLQ believes the studies used by the NCEA to develop a range are representative of the body of studies available from which to derive a slope factor. It is also apparent that USEPA, and science bodies in general, advocate the use of a range of slope factor values from which states should select values for individual application. Therefore, OLQ has concluded that the range of slope factors presented in the NCEA document best represent overall toxicity and is suitable for slope factor derivation. OLQ developed a list of evaluation criteria and, using those evaluation criteria, selected the best study (studies) from within the NCEA range to use in deriving a single slope factor for developing default closure levels. This is similar to what has historically been done to derive slope factors.
Comments on the Draft TCE Document
The comment period for this "Draft" document concluded on March 3, 2006, and a response to the comments document was released November 13, 2006. The TCE Response to Comments Announcement and the TCE Response to Comments Document are both available online.
Contact
If you are unable to access any of the documents on this website, please email IDEMRISC at idem.IN.gov.
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